So underrated. He doesn't get fleshed out too much or get too much screen-time but he has a believable motivation and Mads Mikkelsen gives a great performance. I actually really like Kaecilius.

His past was fleshed out in a comic, exploring him losing his family and seeking meaning in the Master of the Mystic Arts. And after a while, he felt that he had not been fulfilled and sought to know more through other and darker means. If any of that was actually incorporated in the movie, then I bet he would be remembered a lot more than he is.

To this day, I can't quite get over the fact that from all the villains of Doctor Strange's comic history, they choose a lackey of Mordo to be the first villain of his saga. Still, Mads makes this role enjoyable. His tears during his exposition scene, are so genuine, he does hint at a greater story, which is expanded on in the comic mentioned above.

His fall from grace story could make an entire other movie if you ask me, so to just have to rush it off it does disappoint me

I feel like Mads should be given the opportunity to come back, even if for another role (voice acting) or a character with a different appearance (see Gemma Chan).

I agree with @Angel Gro , this character could have easily been named Kaluu instead. But otherwise insignificant to the character.

Kaecilius is pretty underrated, he’s one of those that others will point to as “the evil mirror of the hero who has no other motivation than just being evil and power-hungry.”

This may be true, but I blame that on the film failing to explore his backstory. In the scenes he did have, his reasoning is unique and makes sense. His ultimate enemy is time, and he wants more of it because he did not have that time to spend with his loved ones (this sounds like the plot of Spy Kids 4). And being so disillusioned caused him to turn towards a dark power not out of greed, but out of love.

But this is only something that you could realize if you read the prequel comic. Without it, he just sounds like a generic and mad sorcerer.

What a boring villain. Sure, he had the comic that explained his past, but that didn't stop him from being boring. He got a few food quips in, and the movie was very well-done with a good story, but not because of Kaecilius.